Imagers produced according to the CMOS (“Complementary Metal Oxide Semiconductor”) technology are currently the subject of an increasing number of applications due to their low cost price in comparison with CCD (Charge Coupled Device) imagers. Such CMOS imagers were initially used to produce low resolution image sensors of mediocre quality (for example web cameras). Today, after major investment in research and development, CMOS imagers can compete with CCD imagers.
FIG. 1 represents an example of a micro-module 1 for capturing images using a CMOS imager, intended for example to be mounted into a portable device such as a mobile telephone, a camera or a video camera. The micro-module 1 comprises a frame 2, a lens-holder block 3, one or more lenses 4 and a diaphragm 5 arranged in the block 3, an infrared filter 6 and a base 7. A CMOS imager 9 in the form of a semiconductor chip is disposed on the base 7 so as to receive the light passing through the lenses 4, the diaphragm 5 and the infrared filter 6.
The diaphragm 5 is generally formed by a thin opaque plastic plate having a central orifice 5′ letting the light through, having a fixed diameter. The infrared filter 6 is generally a colored resin plate. It is also known to produce the infrared filter 6 by depositing, on the surface of a glass plate, dark zones (chrome deposit) forming a diffraction grating the geometry of which is determined according to the range of wavelengths to be filtered.
The CMOS imager 9 comprises a plurality of photosites each forming one pixel (not visible in FIG. 1). Each pixel comprises a photodiode and a control and interconnection circuit of the photodiode. The pixels are arranged as an array and a mosaic of red, green and blue filters is distributed over the pixel array, generally according to the Bayer architecture (the cells of a line being alternately red and green, or alternately green and blue). Each pixel is thus covered with a determined, red, green or blue, primary color filter and provides a piece of luminance information about the primary color allocated to it, forming a piece of pixel information.
FIG. 2 is a schematic cross-section of the imager 9 in a region corresponding to three pixels PIX1, PIX2, PIX3. Going from bottom to top, layers 9a, 9b, 9c, 9d, 9e and microlenses L0 (L0-1, L0-2, L0-3) can be distinguished. The layer 9a is a semiconductor substrate in which the imager is implanted. This layer thus comprises the active part of the imager that comprises in particular photodiodes and their associated control and interconnection circuits (not detailed). The layer 9b is formed by a dielectric material that entirely covers the substrate 9a. The layer 9c is a passivating layer deposited on the imager at the end of the CMOS manufacturing process. The layer 9d is formed by colored resins and comprises red, green or blue areas 9-1, 9-2, 9-3 forming the above-mentioned primary color filters, with one color filter per pixel. The layer 9e is an intermediate layer of resin forming a base for the microlenses L0 and providing good flatness. The microlenses L0 are arranged in a so-called “MLA” (“Microlens Array”) with one microlens per pixel.
The lens(es) 4 of the optical set are generally formed in molds by means of a polymer resin that is removed from the molds after a baking step. Another known technique of manufacturing the lenses 4 involves printing polymer resin on a base, then the resin hot creeping to obtain a convex (rounded) face.
Such a micro-module for capturing images has the disadvantage of having a relatively complex structure and of needing a considerable assembly time, increasing its cost price.
In particular, the diaphragm 5 and the infrared filter are additional components needing a dedicated production line and storing, handling and assembly steps.